GTK+, or the GIMP Toolkit, is a multi-platform toolkit for creating graphical user interfaces. Offering a complete set of widgets, GTK+ is suitable for projects ranging from small one-off tools to complete application suites.

GTK+, The GIMP Toolkit, was initially made by the GNU Project for the GIMP but is now a very popular toolkit with bindings for many languages.

Alternatively, GTK+ settings can be configured manually by editing ~/.gtkrc-2.0. A list of GTK+ settings can be found in the GNOME library. To manually change the GTK+ theme, icons, font and font size, add the following to ~/.gtkrc-2.0:

GTK+ 3.x

If you use a GTK+ 2.x based DE, like Xfce, LXDE, gnome-tweak-tool won't work; see FS#23644. You need to installlibrsvg, and set your theme manually in {XDG_CONFIG_HOME}/gtk-3.0/settings.ini (this is usually ~/.config/gtk-3.0/settings.ini. An example settings.ini file:

After this, you need to set the same theme in your DE's appearance configuration tool. There are only a few themes which provide a uniform look for GTK+ 3.x and GTK+ 2.x apps. A few examples:

Adwaita for GTK+ 3 and Advaicium for GTK+ 2

Newlooks for GKT+ 3 and Clearlooks for GTK+ 2

Zukitwo

Elegant Brit

Atolm

Hope

Note: There probably are other themes. Some of these themes are available in the AUR. Also, some of them are not usable as is for displaying a GTK+ 2.x panel (light text over light background), so you need to use the provided panel background.

GTK+ and QT

If you have GTK+ and QT (KDE) applications on your desktop then you know that their looks do not blend well. If you wish to make your GTK+ styles match your QT styles please read Uniform Look for QT and GTK Applications.

Configuration file

Note: See the GtkSettings properties in the GTK+ programming reference manual for the full list of GTK configuration options.

The purpose of this section is to collect GTK configuration settings which can e.g. be used within ~/.gtkrc-2.0.

Enabling Customizable Keyboard Shortcuts

You can customize your GTK applications' keyboard shortcuts (those are called accelerators in GTK terminology) by hovering your mouse over a menu item and pressing your desired key combination. However, this feature is disabled by default. To enable it, set

gtk-can-change-accels = 1

Speed up your GNOME menu

This setting controls the delay between you pointing the mouse at a menu and that menu opening in GNOME. Change this to a setting you prefer. I guess the number is in milliseconds, e.g. 250 being a quarter of a second.

gtk-menu-popup-delay = 0

Reduce widget sizes

If you have a small screen or you just do not like big icons and widgets, you can resize things easily.
To have icons without text in toolbars, use